1. Field of the Invention
The invention relates to a degassing process and device for a worm extruder or the like.
A known worm extruder or the like has a worm housing with a chamber containing the worm or worms and at least one degassing passage opening into the worm chamber. A return delivery is arranged in the degassing passage for returning to the worm chamber material which has penetrated into the degassing passage, the return delivery device comprising a conveyor element arranged within the degassing passage and delivering to the worm chamber.
In the processing of synthetic plastic materials with the aid of a worm extruder or the like (especially a worm kneader or a worm injection-molding machine, each with one or more worms), the problem frequently arises that the material fed to the worm extruder or the like is mixed with water deriving for example from a washing process. In addition or alternatively, the material can contain solvents and diluents such as hydrocarbons, chloro-hydrocarbons, ketones and volatile reaction products such as alcohols and/or esters. These must be removed in order to obtain optimum quality products from the worm extruder or the like. Consequently, they are gassified and the material degassed before supply to the worm extruder or in the worm extruder. In this context the expression "degassing" means both the elimination of residual moisture (by evaporation or vaporisation) and the removal of dissolved gases and volatile constituents. Degassing in separate appliances preceding the worm extruder has the disadvantage of greater construction expense and a possible renewed contamination, especially by moistening of the material, on the way from the appliance to the worm extruder.
2. Description of Art
A degassing device is known for example from the periodical "Plastverarbeiter" (`Plastics processor`), vol. 28, No. 5, 1977, pages 233 to 240. In this case the degassing of the material is obtained within the worm extruder with the aid of a radial degassing passage in the region of the middle of the length of the worm FIGS. 1 and 3). The degassing zone in the region of the degassing passage is preceded by a compression zone. At the transition from the compression zone into the degassing zone, which is at atmospheric pressure, an expansion of the material takes place leading to a gas emission by the material. It is sought to exclude penetration of the material into the degassing passage in that the core diameter is greatly reduced in this region, so that the material does not completely fill out the cross-section of the housing. However this measure has not yet proved adequate to prevent a flow of material into the degassing passage. In order to counter this problem it is known from the initially cited periodical passage to form a degassing pocket into the otherwise cylindrical worm interior chamber, in the region of the degassing passage. Further special developments of the degassing opening in the worm cylinder are known from the periodical "Kunststoffe" (`Synthetic Plastics`) in volume 64, 1974, pages 175 to 177 FIG. 3). Moreover a so-called By-Pass Extruder is known (for example Wittfoht "Kunststofftechnisches Worterbuch" (`Technical Dictionary of Plastics`), Part 3, Carl Hanser Verlag, Munich, Vienna 1978, page 367) in which a bypass passage of regulable throughflow opening into the degassing zone is provided for pressure regulation. However these known measures are frequently not fully satisfactory, since the precise setting of the material level close below the degassing passage opening is achievable only with great difficulty if at all. For safety's sake, consequently, the material level beneath the degassing opening will be set lower than absolutely necessary, which reduces the throughput of material through the extruder. In the case of materials of high viscosity, for example pastes, hitherto this so-called cylinder degassing could be applied only to an extremely limited extent, since the material in paste form always penetrates into the degassing passages. This leads firstly to the blockage of the degassing passages and also to a part of the material, namely the material which has penetrated into the passage, no longer being subject to the intended continuous processing and assuming different material properties. If now this branchedoff material is returned, sooner or later, into the worm chamber, a deterioration of the homogeneity of the material results.
In a known degassing device of the initially stated kind the return delivery device consists of a double worm, that is two interengaging worms, arranged in a degassing housing, which rest along their circumference, except the region of mutual overlap, in more or less sealing manner on the internal circumferential surface of the housing interior, which in cross-section forms approximately a figure-of-eight. Now it has appeared that in the case of relatively tough compositions in the range between 0.1 and 10 kPA.s. a completely satisfactory degassing cannot be achieved. Thus what is called flash evaporation can occur, that is a sudden evaporation, in which mass particles of the material to be processed in the extruder are entrained by the gas current, leading to blockage of the return delivery device.